"Define your own data schema" means that you will be able to define the data that will be stored for each repository module you place on a page. Let's say for example, you want to add a repository module to your site to collect/share recipes. You'll start by defining the data elements and properties for each piece of data you want to store. Recipe Name, Directions, Ingredients, a picture of the recipe, etc. and for each data element define the 'type', text, integer, boolean, file, etc. There will be no limit to the schema you can define. Then once you've defined your schema, you will define a series of 'views' ( what used to be referred to as 'templates' ). You can define list views, detail views, upload views, download views.
The forms will support 'scriptable' buttons that will let you navigate from view to view and will have hooks for workflow to provide support for configurable business rules ( not sure if the first release will have a rudimentary workflow engine in it or not, or if I'll wait for the core workflow solution, we'll have to wait on that one )
A cool feature will be that once you define your data schema, the views and your business rules, you will be able to 'package' them up into what's going to be called a 'repository application'. You can then share ( or possible sell ) your repository applications. Someone else can take the package and 'install' it into their repository and they can then have a ready-to-go application.
And, yes, the plan is to support an upgrade from v3 to v4 of the Repository